Proc. of the IEEE International Conference on Smart Instrumentation, Measurement and Applications (ICSIMA) 25-27 November 2014, Kuala Lumpur, Malaysia
A Battery Charge Balancing System with Reducing Inrush High Spike Current for Electric Vehicle Mizanur.R1, Sheroz Khan1, A Rahman2, Mefta Hrairi2, MM Ferdaus3 and Zeeshan Shahid1 1
2
Department of Electrical and Computer Engineering 2 Department of Mechanical Engineering 3 Department of Mechatronics Engineering International Islamic University Malaysia 53100 Kuala Lumpur, Malaysia
[email protected]
[email protected]
Abstract—A charge equalizer system is suggested for use in the future battery-packs employed in plug-in vehicles or house used in UPS-based supply systems deriving energy for supplementing the grid-connected main supply. Such applications are becoming common in Distributed Generation on spot from sources of solar (or wind), especially when grid connected power-supply is interrupted for some reason. The charge equalizer circuit is using resonant circuit, being operated by the switching frequency showing results the effect of when compared to the resonant frequency. The inrush current raises in the switching components of the resonant circuit due to inductor components. This inrush current may damage the switching component and increase the total system cost. In this work, flyback snubber circuit consisting of diode and capacitor has been used for reducing inrush high spike current. In addition, Zero current switching is achieved in this system for reducing the circuit losses. Keywords—BCB; inrush current; flyback snubber; resonant;
I.
INTRODUCTION
Battery packs play an important role in the applications of uninterrupted power supply (UPS) or electric vehicles (EVs). The former is used making continuous supply possible when the main supply is interrupted while the latter are good for use in metropolitan transport by reducing fuel consumption with low carbon emission and ensuring safety and environmental sustainability in the era of advanced technology. The electric vehicle provides better efficiency than the conventional Internal Combustion Engine (ICE). Likewise in Electric Vehicles, series connected battery strings are widely used in Uninterruptable Power Supply (UPS), hybrid electric vehicle (HEV), electric scooter (ESs), electric wheelchairs or Erickshaws, and even in the telecom industry[1-2]. Many types of battery cells are primarily used as the power sources such as Li-ion, Ni-MH, and LiFeO4in these applications. Among these battery cell types, Lithium-ion (Liion) battery cell is the most commonly used because of low self discharge rate, higher energy density and reasonable power capacity, long life-time or cycle and superior non-memory effect [3]. For the sake of long distance driving, the battery cells are charged up and discharged frequently, and may become imbalanced due to natural phenomena such as uneven temperature, internal impedance, aging behavior such as
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chemical degradation [4][5]. As a consequence, the imbalance voltage of cell may lead to causing overcharge and undercharge situation that might ultimately become a risk for causing an explosion or fire and permanently damage respectively[6][7]. Li-ion battery cannot tolerate such problems of the voltage imbalance situation. Therefore, in order to alleviate the problems the Battery Charge Balancing (BCB) system is required. The use of such a balancing system becomes of utmost importance when the corresponding battery packs are used for measurement applications. Many types of charge equalizer circuits and systems already have been proposed, based on passive and active balancing techniques. The passive balancing method is a simple, easy to be implemented and controlled. It is a power consuming method where a resistive element is used for ensuring balancing of charge through power dissipation of the excess energy. However, this method suffers from heat generation and energy loss problem during balancing time [8]. The active balancing is the most powerful technique as it has proven to be very effective and it is associated with less power loss during the time when energy transfer is taking place through the current flowing paths established through switching components of appropriate converter and inverter. These converters based on the style of converters such as DCDC buck-boost, bidirectional DC-DC, ramp, switched capacitor, multi-winding transformer and resonant style converters. All converters consist of inductive elements (magnetic), diode, capacitive, and switching components. Magnetic components (inductor or transformer) used in different types of DC-DC converter. The buck-boost converter is a very simple and easy for implementing charge equalization system by using only the inductor components are used in this converter. However, it has switching loss problem and limited switching frequency and is bulky and heavy [9]. On the other hands, there is no use of the inductor element in the switched capacitor converter (SCC) although it is very simple and cost effective method. However, it takes a long time in having charge balance among the cells. In such switching circuits, the role of PWM signal at a certain duty cycle or frequency and the use of the resonant circuit system for getting high efficiency are very important
issues in resonant converter based autoomatic charge equalization techniques. Soft switching bidirectioonal DC-to-DC converter and quasi-resonant converter have beeen introduced to achieve the zero current switching and soft swittching [10-13]. Resonant switched capacitor converter (RSCC) is an advanced method derived from SCC by adding an innductor with a capacitor to get high efficiency. This convertter is used for output voltage regulating by controlling phase-shhift-control. In [14], quasi-resonant switched capacitor converter is implemented to achieve zero current switchhing and zero voltage-gap in the charge balancing system for two cells. However, this balancing system has inrush highh spike current problem due to inductor components affectingg the switching devices, even the whole system performance. Thherefore, in this paper, a battery charge balancing (BCB) system m is presented for balancing of four cells. Here, a snubber (flybback) diode and capacitor across the inductor components are uused to remove the inrush high spike current and switching lossses inflicted on the MOSFET elements under the zero currrent switching condition to get the best equalization efficiency. II.
Obviously, the pulses shown in Taable 1 are coming from a PWM generator. The PWM turns ON N and OFF the switches, such that current from higher to lower voltage cell is guaranteed to flow until the voltage geets equalized. TABLE I. PWM
PARTICULAR SWITCHESS CONTROLLED BY PWM ON
OFF
S2,S4,S6,S8,S10 and S12
S1,S3,S5,S7,S9 and S11
S1,S3,S5,S7,S9 and S11
S2,S4,S6,S8,S10and S12
CIRCUIT FUNCTIONAL SCHEMA ATIC
The Battery Charge Balancing (BCB) circuuit as shown in Fig. 1 is based on the concept of resonant swittched capacitor converter. The circuit is made of three moduless controlled by twelve switches, S1-S12. Each module consists oof battery cells and LC resonant circuit parallel with snubber circuit that is made of a diode connected in series with a capacitor for removing the inrush high spike current. The moodule1 is made from battery Cells B1 ,B2, switches S1-S4 and a capacitor C1 in series with inductor L1 in parallel coupled with snubber diode D1 and capacitor Cs1. Accordingly, the module 2 is made from battery Cells B3 and B4, switches S5-S8, and cappacitor C2, with supporting component partners of L2 in paralleel with C2 and flyback diode D2. Moreover, the module 3 is made of battery cells c B1, B2, B3 and B4, a capacitor connected with an inductor in series and flyback diode D3 and capacitor Cs3 in parallell with inductor element. Thus three resonant circuits made froom three pairs (L1C1), (L2C2) and (L3C3), all set to functionn such that the charging of capacitor C is complemented by the energy release of its partner inductor L, and vice verse in a seqquential fashion perfectly synchronized by the PWM controllingg the switches. The PWM signal is making the equalization cirrcuit to work in four states; the functional schematics of the fouur modes are as shown in Fig 2. Here, it is assumed to be having Cell B1 with voltage higher than that of Cell B2, and accorrdingly Cell B3 assumed to be with voltage higher than thaat of Cell B4, assuming that voltage of Cell B3 added to that of Cell B4 is higher than the series added voltage of Cell B1, Cell B2. The MOSFET switches S1, S3, S5, S7, S10 and S12 triiggered ON by the PWM positive pulses applied to their gatess. Accordingly, just the opposite happens as instantaneously neegative signals are applied to the gates of MOSFET switches oof S2 (or S1), S4 (or S3), S6 (or S5), S8 (or S7), S9 and S11,as show wn in Figure 3, where Cell B4 is shown to be with the higher vooltage than Cell B3.
(a)
(b) Fig. 1. The proposed Battery Charge Balancin ng (BCB): (a) Block diagram, and (b) Circuit schem matic
The proposed circuit has four modes that explained as follows:
Mode 1 (t2 –t1) : During this time, switches S1 and S3 are on and others off. The circuit acts as LC circuit with step response where an inductor and a capacitor charged up by a battery B1 can be expressed as di 1 + idt + V C ( t = 0 ) = VB 1 V L + VC = L (1) dt C ³ The built up current and voltage can be found as following i(t
- t 1 ) = VB
2
C sin( ω L
1
((t
0
2
(2)
- t1 ) )
The equation (2) can be expressed as i(t
(a)
VB 1 sin( ω 0 ((t Z0
- t1 ) =
2
V C (t
2
- t 1 ) = VB
L C
Where z 0 = 1
ω0 =
(b)
1
2
(3)
- t1 ) )
[1 - cos( ω 0 (t
2
- t1 ) ) ]
(4)
is the characteristics impedance and
is the angular frequency.
LC
Mode 2 (t3-t2): When switches S1 and S3 are (going to switch off) closed after a certain time, but the flowing current through the inductor element cannot change immediately. After switched off, the inductor current will flow throw diode and capacitor in the snubber circuit that removes the high inrush spike current through the switching components. Due to flowing current, the capacitor will also charge. The equation as shown below L
i(t
di dt
= V
d
1 C
+
- t 2 ) = i(t
3
2
³ idt
(5)
- t1 )
(6)
Mode 3 (t4-t3): The switches S2 and S4 are on and others off , the inductor magnetizing current and capacitor voltage start to feed the battery cell B2. The following equations are the flowing current in reverse way through the battery B2, capacitor and inductor. As a result, B2 charged with help of inductor and capacitor. (c)
VB
= VL + VC = L
2
di 1 + dt C
³ idt
(7)
The built up current and voltage can be found as following i(t
4
- t 3 ) = -VB
C sin( ω 0 ((t L
2
4
- t3 ) )
V C (t 4 - t 3 ) = -VB 2 [1 - cos( ω 0 (t 4 - t 3 ) ) ]
(8) (9)
Mode 4 (t5-t4): During turn off the switches S2 and S4, the flowing current through the inductor is not changed suddenly. Therefore, there are some current flown in the circuit. This flowing current causes inrush high spike current that affect the switching devices. (d)
L
Fig. 2. Schematic diagram for one PWM signal: (a) Mode 1 (t1-t2), (b) Mode 2 (t2-t3), (c) Mode 3 (t3-t4) and (d) Mode 3 (t4-t5).
i(t
di dt 5
= V
d
+
- t 4 ) = i(t
1 C 4
³ idt
- t3)
(10) (11)
III.
SIMULATION RESULTS DETAILS
A. Frequency Variations The simulation results are studied under the three conditions depending on whether the PWM switching frequency is lower, equal or higher than the resonant frequency of the proposed Resonant Converter. Some assumptions are considered to simulate the proposed BCB where the resonant inductor is small in size and diode, capacitor and MOSFET switches are ideal, making no losses at all. •
Switching frequency is lower than resonant frequency:
Fig. 4. When the switching frequency is equal to resonant frequency ((fs= 22.5kHz, fr=22.5kHz).
When the switching frequency of balancing circuit under resonant frequency, then the current of the magnetizing inductor in resonant circuit looks like quasi-sinusoidal waveform shown in Fig 3. The zero current switching (ZCS) can be achieved in this condition during the turn on the switches S1, S3 and turn off the switches S2, S3 [15]. Due to having less switching frequency than resonant frequency, the current through the circuit interrupted and balancing process takes long time.
•
Switching frequency is equal to resonant frequency:
According to the conventional resonant converter, resonant frequency should be matched with the switching frequency to achieve maximum efficiency. ZCS can be achieved during both cases: switches (S1,S3) turn on and switches (S2,S4) turn off. Relating to this, if the switching frequency is equal to the resonant frequency, the maximum current will flow through the circuit which leads to having less time to balance the charge among the cells in the battery pack[15]. The flowing current through the inductor look like full sinusoidal waveform and the switching devices operated smoothly shown in Fig 4.
•
Switching frequency frequency:
is
higher
than
resonant
Due to higher switching frequency, current flows through the inductor element, but magnetizing current is not enough to achieve the desired results. However, it makes extra voltage stress on switching devices [15]. The switching devices do not work properly and smoothly due to high switching frequency than resonant frequency shown in Fig 5. This causes circuit power and switching loss.
Fig. 5. when the switching frequency is higher than resonant frequency (fs=34.3kHz, fr=22.5 kHz).
B. Inrush current spike The issue of inductive flyback arises when current through the coil is interrupted. This flyback problem is addressed with the use of snubber circuit operated by the PWM of Fig. 6(a). As is shown in Fig. 6(b), the flyback develops in the form of current flow through S1 and S3. Another point of interest is the zero-current switching (ZCS) ensured using a capacitor in series with an inductor to make up a resonant converter. The resonance phenomenon helps not only removing the inrush spike problem, but also guarantees ZCS when S2 and S4 turn OFF. An advantage of the snubber circuit used in this work is that it does not provide a dissipative path to reduce the peak of the inductive flyback current [as shown in Fig. 6 (d) and (e)].
Fig. 3. When switching frequency is less than resonant frequency(fs = 11.25 kHz, fr = 22.5 kHz) Fig. 6. Removed inrush high spike current in the switching devices.
C. Zero-current Switching It is obvious that during the interval when S1 and S3 are ON, the current flow through S2 (or S4) is zero and vice verse. However, when S1 and S3 turn OFF, making a turn for S2 and S4 to turn ON, current flow through the switches is passing through zero. Alternatively, one say that S1 and S3 are followed by S2 and S4, and vice verse, and this transition of switches takes place with zero current switching as shown in Fig. 7(a), (b) and (c). The zero current switching is another point for making sure reduced power loss.
E. State of charge (SOC) balancing
Fig. 9. The output voltage plots of Battery Charge Balancing for four battery cells when switching frequency is 22.5 KHz that is equal to the resonant frequency.
Fig. 7. Equivalent current flowing graph in different modes (mode1 : t1-t2 , mode2: t2-t3, mode 3: t3-t4 and mode 4: t4-t5) during the charge balancing process.
D. Energy Storage Capacitance The diode and capacitor in series being with the snubber circuit is primarily for protecting the inductive element from harmful spikes produced when current through inductive element is interrupted as shown in Fig. 8(a), (b) and (c). The current through the diode (Fig. 8(b)) is making a gradual rise in the voltage across the capacitor (see Fig. 8(c)), that is, the energy to be dissipated in resistive element is saved in capacitive element.
Fig. 10. The output voltage plots of Battery Charge Balancing for four battery cells at switching frequency 22.5 kHz that is equal to the resonant frequency.
(a)
(b) Fig. 8. Charging the capacitor during inrush current flowing.
Fig. 11. Balancing processing during battery cells (a) charged by external source (b) discharged by load.
F. Experimental Setup The experimental has been conducted for a two-cell circuit consisting of NiMH (Nickel–metal hydride) cells with 2450mAh current rating. The two cells are with 1.356V and 1.114V cell voltage respectively. Four MOSFET switches (IRF540N), 4.7uH inductor, 10uF capacitor used and the switching frequency is 22.5 KHz that is equal to series LC resonant of 22.5 kHz. The following result is slightly different from simulation results of Fig 10 because of each element has individual resistance (0.489ȍ) which mostly affects the results.
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 12. The output voltage plots of Battery Charge Balancing for two battery cells at switching frequency 22.5 KHz.
IV.
[9]
CONCLUSION
In this paper, a battery charge balancing (BCB) circuit for series connected strings has been presented. The proposed circuit has reduced inrush high spike current by introducing a snubber (flyback) circuit across the inductor components in the quasi-resonant switched capacitor converter and shown the effects on flowing current behavior by varying the switching frequency in relation to the resonant frequency of BCB. In addition, the use of snubber circuit is protecting the inductive element from current spikes, which is being saved in capacitor while making sure zero-current switching.
ACKNOWLEDGMENT The authors would like to thank The Ministry of Higher Education (MOHE), Malaysia for MyRA Incentive Grant Scheme (MIRGS)- MIRGS13-02-0003 Entitled: “Development of Green Transportation System. REFERENCES [1]
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